A classic radar countermeasure is the use of chaff. Chaff is employed by distributing thousands to millions of small metal dipoles in the volume being searched by the victim radar. Prior art chaff may be made of a light-weight, electrically conductive material, and may assume the form of stripes of aluminum foil. The large radar cross section produced by the chaff cloud is intended to mask real radar targets (e.g., aircraft) that might be flying in or near the cloud. FIG. 1 shows a ground based radar system 10 that is searching for a jet aircraft 12. The chaff 14, consisting of thousands to millions of dipoles, preferably having a length equal to a half wavelength at the radar frequency, are scattered in the atmosphere and flutter very slowly to earth (on the order of ten hours) due to its light weight. The jet aircraft 12 flies above the cloud of chaff 14 in order to mask its presence from radar beam 11. As shown in FIG. 1, as the radar beam 11 sweeps past this cloud of chaff 14 a very strong reflected signal 13 comes from the multitude of dipoles, as well as reflections from the jet 12 due to leakage of the radar beam through the chaff 14.
As shown by FIG. 2a, the signal return from the jet 12 is shifted by the Doppler frequency given by
      f    d    =      2    ⁢          v      c        ⁢          f      r        ⁢    cos    ⁢                  ⁢    θ  where v is the speed of the jet, c is the speed of light, fr is the radar frequency, and θ is the elevation angle from the radar to the jet. For example, for a jet moving at 1,320 mph (Mach 2 at 40,000 feet), the maximum Doppler frequency at the horizon, θ=0° for a 500 MHz radar is about 1 kHz.
Radar designers try to defeat chaff by using multi-pulse coherent waveforms. See FIGS. 2a, 2b and 2c. The return signals can be Doppler processed (i.e., Fourier transformed into the frequency domain—see FIG. 3) to separate target signals 16, 18 with various Doppler shifts using filters 10- and 10-2. A moving radar target (e.g., jet 14) will have a larger Doppler shift (see spike 18) than the chaff cloud (which drifts at the ambient wind velocity—see chaff spectrum 16). The coherent radar can thus separate the target from the chaff based upon this Doppler shift.
If the radar has Doppler and tracking filtering, as shown in FIGS. 2b and 2c, then the chaff response can be notch filtered (see the filter's characteristic 20), thus bringing the jet's return signal 18 above detection threshold 22 (see FIG. 2c).
The response of the chaff-deploying entity in response to coherent radar processing is to lay more chaff. By dropping an extraordinary amount of chaff, one might hope to either overwhelm the dynamic range of the radar receiver or provide a strong enough zero-Doppler chaff return that significant energy leaks into the higher Doppler bins and competes with the target. This is an inherently inefficient technique as typical Doppler filters may have sidelobes well in excess of −50 dB. Thus, a massive amount of chaff would be needed to reduce the jet's response below the threshold value.
The prior art includes a disclosure by D. P. Hillard, G. E. Hillard, and M. P. Hillard, “Variable Scattering Device,” U.S. Pat. No. 6,628,239, Sep. 30, 2003 and military research programs such as the DARPA Digital RF Tags (DRAFT) program that built active electronic devices that transmitted signals back to interrogating radar systems. The DRAFT tags have a size, weight, cost and power consumption that would make them unreasonable for use in large numbers in an expendable application.